The present invention relates to lubricant compositions for lubricating magnetic data, particularly rotatable magnetic recording media, such as thin film magnetic disks having textured surfaces and a lubricant topcoat for contact with cooperating magnetic transducer heads.
Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk, where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a xe2x80x9chead crash.xe2x80x9d Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by technique generally referred to as xe2x80x9ctexturing.xe2x80x9d Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the texture on the surface of the substrate is intended to be substantially replicated in the subsequently deposited layers.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Alxe2x80x94Mg)-alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11xe2x80x2, a cobalt (Co)-base alloy magnetic layer 12, 12xe2x80x2, a protective overcoat 13, 13xe2x80x2, an a lubricant topcoat 14, 14xe2x80x2. Cr underlayer 11, 11xe2x80x2 can be applied as a composite comprising a plurality of sub-underlayers 11A, 11Axe2x80x2.
The protective overcoat desirably possesses high durability, density and hardness to protect the underlying magnetic layer providing wear resistance and encouraging durability of the magnetic recording medium arrangement. Typically, a thin film of zirconium oxide, silicon oxide or carbon is used as a protective overcoat.
Chromium underlayer 11, 11xe2x80x2, Co-base alloy magnetic layer 12, 12xe2x80x2 and protective overcoat 13, 13xe2x80x2 are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective overcoat to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat increases friction between the head and disk, thereby causing catastrophic drive failure. Conversely, excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and, likewise, catastrophic failure occurs.
The drive towards ever increasing recording density, and faster data transfer rates and the resulting smoother disk surfaces and lower flying heights, has served as an impetus for the development of new lubricants to serve as a lubricating topcoat overlying the protective overcoat. Such lubricants must perform a variety of different purposes requiring diverse characteristics and attributes. For example, the lubricant forming the topcoat is preferably chemically inert, possesses a low vapor pressure, low surface tension, high thermal stability, stability under high shear stress and good boundary lubrication properties. Moreover, it is critical that the lubricant tightly adheres to the underlying surface over the lifetime of the magnetic recording media.
The entire disc surface of a magnetic recording disc, however, is not ideal for reading and writing data. In particular, disc surfaces have asperities, i.e. protrusions on surfaces of the disks, which interfere with the flying characteristics of the data head, as well as the read and write operations of the data head. In operation, the head can come into contact with asperities while the head flies above the surface of the disc. Potentially, this undesirable contact can cause data written to a particular location on a disc to be lost. In an effort to alleviate such occurrences, manufactures commonly burnish the surfaces of disks to reduce asperities located thereon. Typical burnishing processes, however, cause contamination of ceramic oxides, such as aluminum oxide, on the disk""s surface which can catalyze the decomposition of the lubricant topcoat layer resulting in reduced tribological properties.
Several classes of lubricants may satisfy some of the desired properties. Among the many lubricants available, liquid perfluoropolyethers (PFPE) are the most utilized for forming topcoat lubricants on magnetic recording media. PFPE""s have been reported for use as lubricating magnetic media in, for example, U.S. Pat. No. 3,778,308. PFPE having a variety of polar end-groups are known (see, e.g. U.S. Pat. Nos. 3,810,874; 4,085,137 and 4,647,413) and have been used in an attempt to improve adhesion of the lubricant to the magnetic medium (see, e.g. U.S. Pat. Nos. 4,268,556; 4,696,845; 4,889,939; 5,128,216). Their preparation has also been widely reported (see, e.g., U.S. Pat. Nos. 3,810,874 and 5,506,309)
Typical conventional lubricants, such as perfluoroalkylpolyether (PFPE) fluids such as Fomblin Z-DOL, Fomblin TX, and Fomblin Z-Tetraol, etc., generally have 2-4 polar groups at either end of a linear perfluorinated polyalkylether backbone. The functionalized end groups are considered necessary to provide direct bonding, and thus, improved adhesion of the lubricant topcoat to the recording media. Polar functional groups, however, are not necessarily chemically inert and consequently, such conventional lubricants may disadvantageously undergo chemical reactions prior to their application or while on the magnetic medium tending to decrease their adhesion to the disk surface. Undesirable chemical reactions further include degradation of the lubricant itself. Contamination by Lewis acids, such as aluminum oxide, on magnetic recording media further promote degradation of the lubrications.
Thus, a significant factor in the performance of a lubricant topcoat is the ability of the lubricant to resist decomposition over time, particularly decomposition by acid catalysis. Lubricants that can adhere to the surface of magnetic media and resist degradation provide improved tribology and durability.
In view of the criticality of the lubricant topcat, there is a continuing need for improved adherence of the lubricant to the magnetic recording medium, particularly to a carbon-based protective overcoat. There also exists a need for a lubricant topcoat providing improved durability, stiction and wear performance, particularly under conditions of high stress, temperature, and humidity.
An advantage of the present invention is a lubricant composition with improved resistance to degradation, particularly, resistance to thermal and acid decomposition.
Another advantage of the present invention is a magnetic recording medium comprising a lubricant composition exhibiting high stability to degradation.
Additional advantages and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a magnetic recording medium comprising a magnetic layer and a lubricant composition on the magnetic layer, wherein the lubricant composition comprises a first fluoropolyether and a decomposition inhibiting amount of a second fluoropolyether having at least one nitrogen containing end group.
According to embodiments of the present invention, the nitrogen containing end group of the lubricant molecule comprises one or more amine and/or amide groups. It is advantageous to have one or more amine and/or amide groups as end-groups on the second fluoropolyether to reduce catalytic degradation of the lubricant composition.
In an embodiment of the present invention, the second fluoropolyether has the following formula:
Zxe2x80x94(A)q
wherein Z is a fluorinated polyalkylether; A is an amine or amide group; and q is an integer of 1 to about 4.
In an embodiment of the present-invention, Z is a perfluoropolyether comprising a plurality of xe2x80x94(CaF2aO)nxe2x80x94 repeating units, wherein subscript a is independently in each such units an integer of from 1 to about 10 and n is an integer from 2 to about 100 and A is a NR1R2 or CONR1R2 group wherein R1 and R2 are independently H, substituted or unsubstituted alkyl, or substituted or unsubstituted aryl groups.
Another aspect of the present invention is a method of manufacturing a magnetic recording medium. The method comprises forming a magnetic layer on a substrate; and forming a lubricant topcoat on the magnetic layer, wherein the lubricant topcoat comprises a first fluoropolyether and a decomposition inhibiting amount of a second fluoropolyether having a nitrogen containing end-group.
Another aspect of the present invention is a lubricant composition comprising a first fluoropolyether and a decomposition-inhibiting amount of a second fluoropolyether, wherein the second fluoropolyether has a nitrogen containing end-group.
Embodiments include a lubricant composition wherein the first fluoropolyether is a perfluoropolyether; the second polyether is a fluoropolyether having an amide group, such as a CONR1R2 group wherein R1 and R2 are independently H, substituted or unsubstituted alkyl, or substituted or unsubstituted aryl groups; and the composition comprises no less than about 20 weight percent (wt. %) of the second fluoropolyether.
Additional advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following detailed description, wherein the embodiments of the invention are described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.